1. Field of the Invention
The present invention relates to an injection apparatus, particularly an injection apparatus preferably used for injecting reinforced plastics or bulk molding compounds (BMC) such as thermosetting resins including glass fibers.
2. Description of the Related Art
BMC, such as unsaturated polyester resin containing glass fibers, have been provided having various compositions and properties as needed for specific usages and molding processes. In molded products of BMC, the mechanical properties have a correlationship with the states of the fibers in the products, and the mechanical strength of the products is enhanced as the fibers are longer and are not bent. The fiber length in the product is related to not only the starting glass fibers but also to a molding process per se. In this regard, it is desired to adopt a molding process which causes the least possible damage to the glass fibers therein, in order to improve the mechanical properties of the product.
A plunger type molding apparatus does less damage to the glass fibers, and less bending of the fibers occurs, compared with a screw type molding apparatus. However, the use of such plunger type molding apparatuses has decreased, in spite of the above advantages. This is because the construction of the plunger type molding apparatus is disadvantageous, as explained hereinbelow with reference to FIG. 2 and FIG. 3 attached hereto.
Referring to FIG. 2 showing a conventional plunger type injection apparatus, the apparatus 1 comprises a stuffer or feeder 3 and a barrel 4 connected thereto by an elbow conduit 2. The stuffer 3 is provided with a feed cylinder 5 having a piston which forms a plunger 6 disposed in the stuffer 3 for reciprocating movement. An injection cylinder 7 is provided with a piston forming an injection plunger 8 slidably disposed in the barrel 4. The barrel 4 has an injection nozzle or orifice 10 formed at the free end thereof and a feed inlet 9 for admitting material from the stuffer 3 opening at an intermediate portion thereof in a position relatively close to the nozzle 10. A ball 11 is provided to form, in combination with the feed inlet 9, a check valve for preventing a back flow of the material. With the above arrangement, materials 12 are charged from the stuffer 3, and the plunger 6 is forced to move forward by the feed cylinder 5. The materials 12 are forced to pass through the elbow 2 and enter the barrel 4 through the feed inlet 9. While the supplied materials in the barrel are prevented from flowing back to the stuffer 3 by the check valve having the ball 11, the injection cylinder 7 is actuated to force the injection plunger 8 to move forward, to thereby inject the materials into a mold (not shown).
FIG. 3 shows another conventional plunger type apparatus in which the same numerals denote the same members or elements as those of the apparatus in FIG. 2. The apparatus shown in FIG. 3 has some differences from the first apparatus as shown in FIG. 2, in that the corresponding feed inlet 9A opens to the barrel 4A, at the other end thereof far from the nozzle 10, and a check valve is not provided.
According to the apparatus shown in FIG. 2, it is noted that the feed inlet 9 is located close to the nozzle 10, with the result that stable metering of the materials 12 is easily effected since the materials can be supplied into the barrel 4 in a metering process, while being subjected to a back pressure exerted by the injection cylinder 7. In turn there is a disadvantage in that the materials 12 are likely to remain or stay in the barrel 4, since a leading part of the materials is likely to advance toward the injection plunger 8 and a following part of the materials is forced to move toward the nozzle 10, with the result that the subsequent materials are injected before all of the preceding materials are injected. Further, a sealing of the plunger 8 is likely to be damaged since the sealing must pass over the feed inlet in the injection process
The apparatus as shown in FIG. 3 has certain disadvantages in that metering of the materials 12 is unstable, and the involvement or entrainment of air in the materials is likely to occur. This is because, with the feed inlet 9A provided at the side of the injection plunger 8, each shot is carried out at each retirement of the injection plunger 8, with the result that a leading part of the materials entered in advance must be ejected from the nozzle 10 before the following part of the materials, but the injection cylinder is not allowed to exert a back pressure against the materials in the barrel.